External electrodes of a ceramic chip part are generally produced by the following process: Powder of a conductive material, e.g., silver (Ag), and powder of a glass frit are dispersed in an organic vehicle comprising an organic solvent and a binder resin dissolved therein to thereby prepare a conductive paste. The conductive paste is then applied to a ceramic chip body comprising internal electrode layers separated by dielectric ceramic layers. The applied conductive paste is dried, and then baked to remove the organic substances contained therein to thereby form a conductive film composed of the conductive material and the glass frit. A film of nickel (Ni) and a film of either tin (Sn) or solder are then formed successively on the conductive film by plating to thereby form an external electrode.
In the above process, lead glass, zinc glass, borosilicate glass and the like are used as the glass frit contained in the conductive paste.
In the conventional production process of an external electrode, however, problems have frequently arisen because of the penetration of a plating liquid into the ceramic chip body during the plating step after the baking of the conductive paste. Specifically, the multilayer chip part has problems, for example, that it has internal defects such as delamination and cracks, that the external electrode of the chip part has a reduced adhesion strength, and that after the chip part is mounted on a circuit board substrate, it is apt to suffer breakage due to warpage of the circuit board.
In order to overcome these problems, investigations have been made on techniques for inhibiting the penetration of a plating liquid into a ceramic chip body by increasing the amount of the glass frit contained in a conductive paste or by employing a glass frit having a high melting point which is chemically stable.
However, the recently developed ceramic chip parts produced by using lead oxide series materials, e.g., high-capacitance, small, multilayer ceramic capacitors with excellent bias properties, have the following problem: The ceramic material employed in such a capacitor is highly reactive to the glass frit. Hence, although a conductive paste containing the glass frit having a high melting point is applied and baked, the ceramic material is denatured by reaction with the glass frit. As a result, not only the intended capacitor properties previously attained are lost, but also the plating liquid is liable to penetrate into the resulting ceramic chip body to cause deterioration of capacitor properties.